1. Field of the Invention
The present invention relates to a novel prosthesis for the knee joint. This prosthesis is more particularly intended to include an arrangement comprising means for optimum contact, stabilization and guidance between the movable components during movements in three directions.
It will be recalled that a knee prosthesis is an implant intended to surgically replace a destroyed knee joint.
A knee prosthesis includes elements integral with the bone, and generally made of metal, and sliding elements, generally made of a plastic material such as polyethylene. The metal elements are:                a femoral component which more or less reproduces the form of the femoral articulation of the knee; it is fixed to the bone, either directly via a rehabilitation surface, or indirectly by means of an acrylic resin;        a tibial surface fixed to the upper end of the tibia, either directly, or by means of an acrylic resin; and        a patellar surface fixed to the posterior face of the patella, either directly or by means of an acrylic resin.        
These metal components are generally made of  chromium-cobalt, of titanium or of stainless steel, or of other metals and alloys, in particular ceramics.
The sliding and shock-absorbing components separating the metal components are generally made of polyethylene; they can either be made integral with the tibial component or fixed plateau or can be movable relative to the latter with a variable degree of mobility in rotation (horizontal plane), in antero-posterior displacement (sagittal plane), and in medio-lateral displacement (frontal plane); whether they are fixed or movable, the polyethylene sliding components called “inserts” articulate with the femoral component via two femoral condyles, an internal one and an external one, these condyles having a certain congruence with the two respective concave surfaces, called glenoid cavities, of the polyethylene insert. A prosthesis is said to have a movable “plateau” when the insert has a free axial rotation. The prosthesis according to the invention is preferably of this type.
The free axial rotation is a corollary of the congruence; congruence is a major tribologic advantage by virtue of which the pressures and the wear on the polyethylene constituting the insert are low, which gives the prosthesis a longer useful life.
In more recent forms of prostheses, the bearing is effected via three surfaces, a third bearing zone situated between the two condyles being in contact with a median projection situated on the upper surface of  the insert. The engagement of these two parts can involve:                either a third femoral condyle engaging in a third cavity on the upper face of the insert;        or a stud on the upper face of the insert engaging in a cavity (nest) situated between the two femoral condyles;        or different forms of cams situated between the femoral condyles and bearing on an elevation on the upper part of the insert.        
In all cases, however, there are angulations in a frontal plane, rendering the contact between the components discontinuous.
The anterior part of the femoral prosthesis (trochlea) articulates with the patella by way of a polyethylene medallion. This polyethylene medallion is fixed to the patellar seat or can be movable relative to the latter. There is a certain congruence between the articulation of the femur and the articulation of the patella.
A knee prosthesis does not have to reproduce the exact forms of an anatomical articulation. The reason is that, in the latter, the menisci adjust the congruence between the femoral and tibial components. The cruciate and collateral ligaments co-adapt these components and ensure, by cooperation with the asymmetrical form of the articular surfaces, a relative displacement in the three spatial planes (frontal,  sagittal and horizontal). As these different anatomical elements are absent in the prosthetic knee, the latter has to satisfy three types of requirements: the physiology requirements, the tribology requirements, and the stability requirements.
(1) Physiology Requirements
The physiology requirements of the knee must be respected, in particular the displacements and respective positions of the femur relative to the tibia: rolling/sliding with retreat of the point of contact of the femur on the insert during flexion (90°), and advance of the femur during extension (0°), which conditions the moment of action of the patellar tendon, which controls the efficacy of the muscular propulsion and, by this means, the comfort of the patient and the quality of walking, of going up and down stairs, and of standing up from a low position. These relationships must be ensured, during the movements, by the form of the articular surfaces of the prosthetic components (determined by the design of the prosthesis) and in particular by a cam effect between the femur and the insert, in cooperation with balanced tensioning of the collateral ligaments of the knee joint, determined by the action of the surgeon and by ancillary instrumentation respecting this requirement.
(2) Tribology Requirements
The tribology requirements must be respected so that the contact between the femoral components made of  metal and the tibial and patellar inserts made of polyethylene do not generate phenomena of wear and tear of the polyethylene. In particular, the contacts must be as congruent as possible, since punctiform or linear contacts or contacts over a small surface generate high pressure in the polyethylene and thus wear and creep of the latter.
2. Discussion of the Prior Art
In certain types of prostheses, the polyethylene insert is fixed relative to the tibia. In this case, the tangential rolling/sliding movements, the shearing movement and all the movements of rotation take place in the contact zone between the femoral condyles and the tibial glenoid cavities. When these contacts are over a small surface area, high pressure can be reached in the polyethylene. Their designers generally recommend a pressure of less than or equal to 10 Mpa in the polyethylene zones where bearing is constant, that is to say in the zones near the walking position. An ideal pressure of 4 Mpa is desirable. However, in the prostheses with a low congruence (linear of punctiform articular surfaces between femur and polyethylene insert), pressure of over 30 Mpa are currently measured, and can even reach 50 Mpa. This leads to rapid degradation of the polyethylene, possibly necessitating further surgery to change the prosthetic implants. A contact surface of greater than 400 mm2 is recommended. The state of the art, in the field of prosthetic joints, has led to the manufacture of prostheses in which the polyethylene inserts are congruent in relation to the femoral surface, a corollary of this congruence being that the insert must be movable in rotation relative to the tibial seat. These implants are referred to as prostheses having a movable plateau. In such cases, the articular congruence can be respected, making it possible to obtain a high degree of surface contact. With this type of prosthesis, pressures of the order of 4 to 8 Mpa are currently obtained in the polyethylene, which promotes the useful life of the latter. 
Various congruent prostheses exist, but they all have two major disadvantages, which the present invention aims to correct:                the congruence of the contact between the condyles and the tibial glenoid cavities is present particularly near extension but diminishes during flexion;        the congruence is envisaged in only one plane: the sagittal plane; thus, considerable stresses can arise in the contact zones during movements of inclination or lift-off, or rotation, or in combination in these three directions, particularly when the surfaces include angulations.        
To obviate this, the various surfaces must be tangential in relations to one another in the two spatial planes (sagittal and frontal) in order to permit sliding movements without sudden stops and without angular contacts in these three directions. 
(3) Stability Requirements
The stability of the knee in which a prosthesis is fitted can be respected by different mechanisms:                retention of the two cruciate ligaments;        retention of only the posterior cruciate ligament, although it has been shown that in this case the antero-posterior displacement does not have satisfactory kinematics;        finally, stabilization of the prosthesis by a central mechanism intended to guide the femur in relation to the tibia.        
This latter type of mechanism is in fact the most effective for keeping the stability in accordance with the kinematics of the knee, and it is a device of this type with which the present invention is concerned.
The state of the art in this field includes several types of mechanisms:                the most traditional is that of a rod which comes into contact with a vertical central stud of variable shape;        other mechanisms use a cam cooperating with a central abutment which has a profile complementing this cam; for example, the third and central femoral condyle; situated between the two lateral condyles;        in other prosthesis configurations, there can  be a third, central condyle which itself is in the form of a transverse cylinder which is engaged in a transverse cylinder ending at its front part in the form of a cam.        
A disadvantageous effect of this latter configuration is a total absence of freedom of antero-posterior movement between the femoral component and the tibial insert when the cylinder has the same transverse axis as the two lateral condyles. It is therefore a mechanism of the so-called hinge type, which can be responsible for considerable pressures in the articulation between the patella and the femur. The mechanism of rolling/sliding is not respected and one of the consequences can be the existence of considerable patellar pain. The recovery of a certain forward/rearward mobility in the zone of contact between polyethylene insert and tibial plateau is not sufficient to overcome this disadvantage; this mobility situated at a different level than normal generates a parasite movement referred to as “roll forward”, during which the tibia moves back in flexion relative to the femur, which can lead to disengagement of the prosthetic components.
In a configuration of this type of prosthesis (see WO 98/46171 A), the median projection has on its upper face a guide curvature, bearing on a complementary surface situated between the two femoral condyles producing a cam effect, which develops an  optimum displacement of the femur relative to the tibia. However, the engagement of this device in the frontal plane reproduces a broken curve, a source of shocks, of angular contact (edge contact), sudden stops, and, thus, mechanisms which can cause loosening of the prosthesis.
The present invention is therefore based on a prosthesis with movable plateau, the plateau or insert being totally congruent in the transverse direction via its upper surface with the articular surfaces of the femur and, via its lower face, with the upper face of the metal seat implanted in the tibia.
Among the currently existing prostheses with congruent movable plateau, most have a congruence in the zones near extension, but this congruence diminishes considerably upon flexion on account of the progressive decrease in the radii of curvature of the condyles in the posterior part of the articulation, whereas the radius of curvature of the tibial plateau remains constant. This is due to the polycentric radius of the condyle in the sagittal plane.
Other implants have large contact surfaces from extension to flexion. However, in these designs, the different sagittal planes (in the plane XY) are offset in relation to one another, which, when seen from the front, translates into a broken line which is likely to generate high stress peaks in the polyethylene, as well as transmission of abnormally high stresses, shocks and  vibrations to the sites of fixation of the prosthesis during movements of lift-off/lateral inclination.
In general, the bearing of knee prostheses is effected by the contact between the femoral condyles and the tibial glenoid cavities, with a medial bearing and a lateral bearing. Since the mechanical axis of the body running from the center of gravity of the body to the contact of the foot with the ground passes medially in relation to the knee joint, the bearings on both the medial and lateral compartments of the knee are asymmetrical, creating several sources of problems.
One of these lies in the fact that a loosening moment is created by means of a compression in the internal compartment which is much greater than the compression in the external compartment. For this reason, the external compartment of the knee tends to lift at the level of its tibial fixation and lead to a possibility of separating of the latter, the source of mobilization of the prosthesis, which can result in deterioration and a new surgical procedure.
Another source of problems lies in the fact that, between the bearing phase and the oscillating phase of walking, there is a separating of the femoral metal component relative to the polyethylene insert, which in English is referred to by the term lift-off. This lift-off is seen principally between 40 and 70° flexion of the knee, principally in the zone from 50 to 70°. This generally involves the separating of the  lateral condyle relative to the lateral tibial plateau. This lift-off, which exists physiologically (about 1.8 mm) can attain amplitudes of several millimeters, sometimes 5 mm or more, in the case of certain prostheses.
In the form of bearing elaborated according to the invention, by means of concave-convex engagements in the frontal and sagittal planes, the bearing is effected via the lateral condyles and on the central dome, and on the oblique lateral slopes of this dome, which, as will be seen below, is one of the main originalities of the invention.